Cetaceans and floating marine macro-litter in the Calabrian Ionian Sea (Central Eastern Mediterranean): applying conservation research

1 Introduction

Marine litter is a primary anthropogenic threat to coastal ecosystems and pelagic megafauna [United Nations Environment Programme (UNEP), (2021); Fossi et al., 2018]. Plastic accumulation, in particular, is now recognized as a critical global challenge. Its impact is comparable to other major environmental crises, including climate change, ocean acidification, and biodiversity loss [United Nations Environment Programme (UNEP), (2021); Yu et al., 2023; Karak et al., 2022]. According to the global analysis conducted by UNEP, plastic accounts for at least 85% of total marine debris and constitutes the most persistent and harmful fraction for marine ecosystems. The accumulation of plastic in the Italian seas has devastating effects on numerous benthic and pelagic species, altering ecological dynamics and compromising habitat functionality (Bottari et al., 2024). The dispersion of debris follows complex pathways, influenced by currents, winds, and morphological features, which determine critical accumulation zones for many marine species [United Nations Environment Programme (UNEP), (2023)]. Interactions between marine fauna and plastic debris vary depending on the type and size of the litter and the marine fauna can be harmed through various mechanisms: entanglement, for example, is one of the most visible forms of impact, with effects ranging from physical injury to death by drowning or starvation. Objects such as discarded fishing nets, plastic bags, and rings are among the primary causes of these incidents (Fauna and Flora International, 2023). The ingestion of plastic represents another significant risk. Marine animals, mistaking debris for food, can suffer from intestinal obstructions, malnutrition, or intoxication. Furthermore, ingested microplastics can carry persistent chemical contaminants, such as PCBs and heavy metals, which bioaccumulate along the food chain, with potentially serious effects for apical predators and human health [United Nations Environment Programme (UNEP), (2021)]. Finally, plastic debris also acts as a vector for the dispersal of alien invasive species, which settle on floating surfaces and are transported to new habitats, altering local ecological balances (Bottari et al., 2024). This phenomenon is particularly worrying in areas with high biodiversity, such as the Mediterranean, where anthropogenic pressure and debris density are high. Cetaceans represent key indicators of ocean health, and interaction with marine debris is now widespread globally (Fossi et al., 2018). The main documented threats are entanglement in abandoned nets and lines, and the ingestion of plastic. Long-term observations have shown that some cetacean species regularly interact with single-use plastic items, often mistaking them for prey or using them as toys, with potentially lethal consequences (Rodríguez et al., 2023). It is estimated that over 66% of cetacean species are affected by interactions with marine debris, with effects ranging from chemical contamination to the impairment of vital functions (Fossi et al., 2018). Floating marine litter (FML), particularly macro FML (>2.5 cm) poses a persistent threat to marine megafauna, as it remains suspended in the surface layers of the ocean where it interacts with numerous pelagic species. Floating dangerous debris for cetaceans (DDFC) is a high-risk subcategory of floating marine litter (FML) that specifically promote harmful interactions with megafauna. Within this category, the objects most frequently associated with hazardous interactions include fishing gear (nets, lines, ropes), large plastic objects, textile and synthetic materials, and other entanglement-prone debris such as straps, bands, and balloon strings (Sá et al., 2021). Vulnerable species, such as whales, dolphins, and sea turtles, are particularly prone to both ingesting and becoming entangled in these types of debris. Because DDFCs persist on the surface, they frequently overlap with the habitats of surfacing breathers (Lebreton et al., 2018). To address the need for effective conservation tools, this study integrates spatial modeling of DDFC distribution with biological and oceanographic data.

The objective of this study is to identify areas of high ecological vulnerability by quantifying the spatial overlap between debris and cetaceans. This approach aligns with the marine strategy framework directive (MSFD 2008/56/EC), and provides a direct method to assess good environmental status (GES) through the prioritization of DDFC monitoring.

2 Materials and methods

2.1 Study area

The Gulf of Squillace, located in the Calabrian Ionian Sea, constitutes the study area where scientific research and marine monitoring activities on cetacean species have been conducted since 2019 (Figure 1). The survey coverage area amounts to approximately 450 km². Among the species observed, the presence of the common bottlenose dolphins Tursiops truncatus (Menniti et al., 2023), the Risso's dolphins Grampus griseus (Menniti and Vella, 2022), and the striped dolphins Stenella coeruleoalba has been recorded as regular, while other species are less frequent visitors. The Ionian Sea acts as a hydrodynamic crossroad between the western and eastern Mediterranean basins, connecting to the Adriatic Sea via the Strait of Otranto. Its surface circulation is driven by thermo-haline processes, air-sea fluxes, and wind-induced coastal upwelling. These factors combine to generate significant seasonal and interannual variability (Menna et al., 2021). Recent studies based on three-dimensional hydrodynamic models (e.g., SINTEF Model—SINMOD) and satellite data (Copernicus, Argo floats) have highlighted that the Ionian Sea features dynamic convergence and divergence zones. These conditions favor the accumulation of floating marine litter and, consequently, the formation of critical habitats for cetaceans and marine turtles (Lebreton et al., 2018; Maximenko et al., 2019).

2.2 Sampling methods and data collection

The research activities were conducted using a transect sampling design within a geographically defined study area, following a bathymetric profile between 50 and 800 m in depth. Cetacean monitoring has been conducted since 2019, while a specific floating macro-debris monitoring campaign carried out in 2024–2025. Seasonal dynamics—such as changes in currents, tourism pressure, biological cycles, and weather conditions—can significantly alter the quantity, type, and spatial pattern of floating debris, as well as the detectability and habitat use of marine fauna. For this reason, floating macro-debris monitoring campaign explicitly covered two well-defined summer periods: 2024, from 14 July to 11 September and 2025, from 3 June to 2 September. The time intervals identified allow for comparable seasonal conditions and support more robust interpretation and comparison over time. Surveys for cetaceans and other protected species and marine litter assessment were conducted using an 8 m inflatable boat, under favorable marine weather conditions (Beaufort Scale ≤ 3) and good visibility (≥2 km). The sampling effort was set at about 5 h/day, maintaining a cruising speed between 13 and 15 Km/h, which was reduced during sightings. The scientific team onboard included at least two researchers/photographers and two experienced observers. The observation arc of 360° was divided into two equal sectors, each monitored by an observer. Observers conducted continuous naked-eye scans, utilizing 7 × 50 binoculars primarily to confirm the identification of debris or species detected during the initial scan and to survey the horizon at 5-min intervals. The geographic coordinates of the boat were automatically recorded every 1 min using a GARMIN ETREX 10 GPS. The navigation log was filled out every 20 min or whenever a sighting required reporting.

2.2.1 Cetacean detection

During cetacean sightings, a minimum approach distance of about 100 m was maintained to avoid crossing the cetacean's path, unless the cetaceans themselves attempted to approach. Recorded data included: date, sea-weather conditions, geographic coordinates, depth (m), time of start and end of sighting, sighting sequence number, best estimate of group size and composition, behavior (activity patterns were recorded according to Altman and Shane as “traveling,” “foraging,” “socializing,” and “resting” and they been recorded continuously) (Altman, 1974; Shane, 1990). Group size estimation was defined as the min-max and best estimation, corrected a posteriori using photo-identification analyses. Images and videos for photo-identification were collected using a Nikon D750 digital camera equipped with a Nikon AF–S Nikkor 28–300 mm 1:3.5–5.6 G lens and were recorded perpendicular to the longitudinal axis of the animal (Würsig and Jefferson, 1990).

2.2.2 Marine litter detection

During the cetacean sighting surveys, other ecological information, including the presence of other protected species, fishing activities, and the presence of marine litter were also collected. The approach for Floating Marine Litter (FML) observations was as follows. Precise GPS coordinates were registered, and for each FML observation, the number and description of the floating litter were recorded. Although macro-litter is conventionally defined as items exceeding 2.5 cm, and despite the fact that surveys were conducted under optimal visibility conditions, it is probable that objects within the 2.5–5 cm range were underestimated. Due to the inherent limitations of visual detection from a moving vessel, these smaller items may have been overlooked. For operational efficiency, floating litter that did not pose a risk to the inflatable boat was retrieved and subsequently analyzed, enabling the identification of its material composition. To ensure consistency and comparability across Member States' monitoring programs, the European Commission developed a harmonized methodology for the classification of marine litter, known as the Joint List of Litter Categories (J-List) (Fleet et al., 2021). This list is available on the official portal of the Marine Litter Technical Group.¹ The J-List provides a hierarchical and standardized classification of marine litter, divided into main categories, subcategories, and identification codes. It is designed to be applicable to different environments (beaches, water column, seafloor) and various collection modalities (direct observation, sampling, remote sensing).

The key methodological features include:

• Unique litter coding: each item is associated with an alphanumeric code that identifies its typology (e.g., J8 for plastic drink bottles >0.5 l, J45 for fishing nets, specifically plastic mussels/oyster mesh bags, net sack, socks).
• Visual support: the list is accompanied by photographic examples to facilitate field identification.
• Adaptability: the structure is compatible with regional lists (e.g., OSPAR, UNEP/MAP) and can be integrated into citizen science projects and automated monitoring.
• Alignment with MSFD criteria: specifically with criteria D10C1 (quantity and composition of litter) and D10C2 (impacts on marine species).

The classification of the observed floating dangerous debris for cetaceans (DDFC) on the surface was carried out out according to the J-List, by selecting those categories (e.g., fishing gear, lines, plastic bags) that are known to pose the highest risk of entanglement and ingestion to cetaceans, in line with the methodological approach for spatial risk exposure assessment in the Mediterranean Sea (Arcangeli et al., 2024). Moreover, to assess the potential risk posed by DDFCs, the corresponding encounter rate was derived to quantify the number of detected items per kilometer surveyed, thereby standardizing observations relative to the applied sampling effort.

2.3 Data analysis

All geospatial processing and distribution analyses were performed using the open-source GIS software QGIS² (version 3.14.16).

The GPS data recorded every minute during the transects were used to generate the effort tracks, calculated as the total transect length in kilometers covered under on effort conditions. Cetacean sightings and floating marine litter (FML) were georeferenced and mapped across the study area, but only debris categorized as DDFC was included in the analytical assessment. FML observations were classified by litter category.

About categorical quantitative analysis for marine litter, sightings were classified and aggregated according to the joint list of litter categories (J-List) (Fleet et al., 2021). QGIS was utilized to visualize the spatial distribution of the different J-List codes, highlighting areas of higher concentration for specific materials. A spatial overlay analysis was conducted to simultaneously visualize the cetacean sighting tracks and the DDFC points, allowing for an assessment of their spatial co-occurrence to understand possible geographic risks. To quantify abundance and allow for spatial comparison, we calculated the encounter rate (ER) for both target groups. The Cetacean ER was defined as the number of sightings per on effort kilometer traveled (S/km), while the DDFC Encounter Rate was calculated as the number of dangerous debris per on effort kilometer traveled (DDFCs number/km). High-risk areas were identified by integrating these datasets through a standardized spatial grid. Specifically, a spatial overlay analysis was conducted to assess the co-occurrence of cetaceans and DDFCs. The impact risk was quantified by deriving the DDFC ER specifically along the track segments where cetacean sightings occurred (DDFC/Cetacean sightings tracks ER). The resulting overlap offered a quantitative basis for delineating vulnerability zones.

3 Results

3.1 Survey results

During the 2 year survey 2024–2025, 34 field surveys were conducted during summer seasons, covering a total of about 1,410 km of transect survey effort (Figure 2), along the coastal area from Catanzaro Lido to Santa Caterina dello Ionio. The effort was distributed across the bathymetric profile between 50 and 800 m, as designed. The monitoring resulted in 26 sightings of the following species: five sightings of bottlenose dolphins Tursiops truncatus; 18 of striped dolphins Stenella coeruleoalba and three of Risso's dolphins Grampus griseus (Figure 3 and Table 1). The sighting tracks of the species encountered were: about 42 km for striped dolphins (an observation was recorded as a point-based track, in which the starting point coincides with the ending point), 17 km for Risso's dolphins and 10 km for common bottlenose dolphins where two observations were recorded as a point-based track (Figure 4). Research effort distribution was mapped across a grid composed of 258 monitored cells, each of 1 km². A total of 37 cells recorded a Cetacean ER value greater than zero. The overall minimum and maximum Cetacean Encounter Rates observed across the study area were 0.05 and 0.74. The highest encounter rate values was recorded at a distance of approximately 10 Km offshore from Soverato, in a depths range between 700 and 800 m, ER values ranged from 0.61 to 0.74. Other medium to high ER values were observed in two main areas: offshore Stalettì-Squillace, recording an ER of 0.48 in depths ranging between 200 and 500 m; offshore Soverato-Satriano, recording an ER of 0.49 in depths between 700 and 800 m (Figure 5).

Table 1. Summary of cetacean sighting records (2024–2025).

Regarding FML, the monitoring resulted in 86 sightings categorized according to the J-List codes specified in Table 2. The data analysis identified 23 waste typologies. The predominant fraction consists of artificial polymer materials (letter code “pl_”), accounting for 96.5% of the total waste recorded. A minor share of metallic waste (letter code “me_”) was observed, representing 3.4%, together with a single occurrence of cloth/textile waste (letter code “ct_”; Figure 6). The distribution of observed waste highlights a marked predominance of plastic items, with 83 records. Within this category, the most frequent objects are plastic shopping/carrier/grocery bags, which alone account for 37% of the observations, followed by plastic toys and party poppers, plastic drink bottles greater than 0.5 L, and fish boxes made of foamed polystyrene. The remaining plastic typologies (nets, gloves, buckets, packaging, etc.) were recorded at very low frequencies. The metal category (me_) is represented by three observations, including beverage cans, fishing weights, and other metallic pieces greater than 50 cm. All debris documented during the survey was classified as floating dangerous debris for cetaceans (DDFC). Consequently, the 86 identified objects were considered potentially hazardous to cetaceans due to their potential to cause entanglement, collision, or other harmful interactions. The DDFC encounter rate, calculated on the presence of dangerous debris items and on effort tracks, shows a total of 58 cells recorded with a sampling effort greater than zero. The overall minimum and maximum DDFC encounter rate values observed across the study area were 0.05 and 1.23. The highest value (1.23) was recorded at a distance of approximately 10 Km offshore from Sant'Andrea Apostolo dello Jonio, in a depth range between 600 and 700 m. Another high value was observed offshore from Badolato, recording a DDFC ER of 0.92 in depths ranging between 100 and 300 m (Figure 7).

Table 2. Summary of litter recorded, including the official name from the MCC/JRC catalog, corresponding letter-code, and total number of sightings.

Five different intervals were extrapolated from the overlap between cetacean sighting tracks and DDFC observations (DDFC-Cetacean sighting tracks encounter rate; Figure 8). From the results obtained, it is possible to identify a shared area, at potential spatial risk, with a depth between 200 and 700 m, located between Squillace and Davoli. As with the calculation of the distribution of cetacean research efforts, this overlap ER also mapped on a grid of 258 monitored cells, each 1 km across. In total, 18 cells recorded an overlap Encounter Rate value greater than zero. The highest value (overlap ER = 20), was recorded in the offshore area of Soverato, at a distance of approximately 5 km from the coastline (Figure 9). This peak indicates a high density of DDFC concentrated precisely within the limited tracks where cetaceans were actively monitored, signaling a high localized probability of interaction.

4 Discussion

The 2024–2025 monitoring campaign confirms the Gulf of Squillace as a significant accumulation area for floating marine litter, with a massive predominance of artificial polymers (96.5%). This high plastic concentration aligns with broader Mediterranean trends documented by Di-Méglio and Campana (2017), who observed similar patterns along the French coast. Our results show that plastic accumulation is not limited to nearshore areas; the high DDFC ER of 1.23 recorded 10 Km offshore Sant'Andrea Apostolo dello Jonio (600–700 m depth) suggests that offshore hydrodynamics and persistent currents are likely concentrating debris away from the coastline. In contrast, the hotspot near Badolato (DDFC ER of 0.92 at 100–300 m) may reflect more direct influence from coastal sources and riverine inputs. Notably, 100% of the recorded items were classified as DDFC. The prevalence of plastic bags and fishing-related items underscores a dual pressure from land-based activities and maritime sectors, representing a severe threat to the ecological integrity of the Ionian pelagic zone. The regular occurrence of Stenella coeruleoalba, Tursiops truncatus, and Grampus griseus confirms the biological significance of the Gulf of Squillace, particularly between the 200 and 800 m isobaths. Our spatial analysis identified a critical ecological risk hotspot between Squillace and Davoli, where cetacean sightings and DDFC concentrations frequently overlap. While direct physical interactions were not visually captured, the potential impact is well-supported by literature. As highlighted by Panti et al. (2019), marine litter represents a major threat to Mediterranean marine mammals, even when impacts are not immediately visible. The types of debris identified in our hotspots, such as plastic films and nets, are the same materials documented by Alexiadou et al. (2019) as leading causes of fatal ingestion and entanglement for odontocetes in the nearby Greek Seas. The presence of these hazardous materials in the deeper pelagic zones (600–800 m), where striped dolphins and Risso's dolphins are most frequent, suggests a high localized probability of encounter.

A key methodological finding is the discrepancy between the general DDFC encounter rate (max 1.23) and the localized overlap encounter rate (value of 20) recorded offshore Soverato. This high value is a result of a multi-scale analysis: while the general DDFC ER provides a baseline for the entire 1,410 km effort, the Overlap ER quantifies debris density specifically along the short sighting tracks where the animals were present. This standardized approach allows us to “zoom in” on environmental conflicts, providing a more realistic assessment of the risk than a simple average over the entire study area.

5 Conclusion

This study establishes a robust baseline for both cetacean distribution and floating marine litter (FML) presence in the Gulf of Squillace, a Mediterranean sector that, despite its ecological value, has remained poorly investigated regarding anthropogenic threats. The methodology employed, based on the J-List categorization and standardized encounter rates, proves to be an efficient and replicable tool for assessing environmental quality in alignment with the marine strategy framework directive (MSFD 2008/56/EC).

The identification of specific area of concern, where the spatial overlap between cetaceans and floating dangerous debris (DDFC) is most intense, provides an empirical basis for evidence-based conservation. Our findings suggest that:

• Targeted mitigation: conservation efforts should not be distributed generically across the coastline but must be prioritized in the “vulnerability corridors” identified (e.g., the offshore areas between Squillace and Davoli). These zones require urgent debris monitoring and recovery programs and the implementation of “Ghost Gear” recovery initiatives to mitigate the risk of entanglement.
• Management of protected areas: the regular presence of three cetacean species within the SAC “Fondali di Stalettì” and the Marine Park “Baia di Soverato” justifies an update of current management plans. Integrating FML monitoring into the routine activities of these protected areas is essential to ensure a “good environmental status” (GES).
• Source prevention: the seasonal nature of the debris—linked to both riverine inputs in winter and maritime tourism in summer—highlights the need for integrated waste management policies that connect land-based prevention with maritime regulations.

In conclusion, this research provides a practical framework for mitigation. Future research should aim to integrate these spatial data with hydrodynamic modeling to predict the seasonal movement of DDFC hotspots. Ultimately, the transition from monitoring to active management is mandatory to safeguard the long-term survival of pelagic megafauna in this vital portion of the Mediterranean Sea.

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Footnotes

Edited by: Aykut Ibrahim Olcer, World Maritime University, Sweden

Reviewed by: Onur Gönülal, Istanbul University, Türkiye

Soufiane Hasni, Abdelmalek Essaadi University, Morocco

Klaus Wysujack, Thünen Institute of Fisheries Ecology, Germany

Footnotes

https://mcc.jrc.ec.europa.eu/main/dev.py?N=41&O=459

http://www.qgis.org